Continuous Positive Airways Pressure (CPAP) machines are well known in the art for use in the treatment of a number of respiratory conditions, such as sleep apnea and hypopnea, by supplying a continuous positive pressure to a patient's airway while the patient sleeps. A typical CPAP apparatus is programmed with a therapy pressure, and is able to maintain the set pressure (measured either at the mask or at a base unit) during the inhalation and exhalation phases of the breathing cycle. The pressure setting is typically programmed via a control on the unit. Bi-PAP machines will typically vary the positive pressure delivered to the user during the inhalation and exhalation phases of the breathing cycle. Typically, Bi-PAP machines deliver a lower pressure during the exhalation phase of the breathing cycle, to make it easier or less uncomfortable for patients to exhale while using the machine. The Bi-PAP machine is typically programmed with a therapy pressure, which is used as the inhalation pressure, while the exhalation pressure is typically a standard difference from the inhalation pressure.
FIG. 1 shows a schematic view of a typical prior art CPAP/Bi-PAP machine 30. Positive pressure is maintained by regulated blower 40, under control of motor control circuitry 38. Blower 40 supplies a pressurized flow of air to a mask connected via the flexible tube (not shown) to blower 40. Regardless of whether the device is a CPAP machine or a Bi-PAP machine, microprocessor 34, in accordance with normal operating programming stored in memory 36 produces a motor control signal which is interpreted by motor control circuitry 38. Motor control circuitry 38 translates motor control signal 37 into electrical impulses that control the speed of blower 40 to produce the desired pressure through flow element 42 and ultimately to the user of the device. The machine may be equipped with various sensors, such as pressure sensor 44 and flow sensor 46 to aid in the detection of sleep events. Control and programming of the device is accomplished via user interface 32.
Prior art breathing therapy machines may also be equipped with an auto-adjust feature. The auto-adjust feature allows the breathing therapy machine to adjust the pressure automatically in response to the sensed condition of the patient, specifically, in response to sensed apneas, hypopneas or episodes of periodic breathing. In a typical prior art implementation of this feature, the therapy pressure is quickly raised in response to sensed events, and then lowered in a linear manner until further events occur, at which time the cycle repeats. The auto-adjust feature is provided to make use of the machine more comfortable for the user and to avoid providing a higher pressure than is necessary to prevent or reduce events.
FIG. 2 shows a pressure graph produced by a common implementation of the auto-adjust feature. When the device detects a certain density of events (i.e., a certain number of events over a pre-determined period of time), for example, as shown in FIG. 2 as reference number 200, the pressure is aggressively raised as shown by reference number 210, until the events are eliminated or until the density of events is reduced below an acceptable level. Thereafter, the pressure is reduced in constant intervals at a slower rate as shown by reference number 220.
There are two problems with the prior art auto-adjust algorithm. First, the aggressive raising of pressure 210 often causes the delivered therapy pressure to overshoot the pressure needed to reduce or eliminate the events, and the slow rate of lowering the pressure 220 causes the therapy pressure to remain at a this higher level longer than necessary. This tends to cause the patient discomfort and may cause arousals during sleep. Secondly, the lowering of the pressure at the constant rate eventually allows the events to start occurring at a density high enough to cause the aggressive raising of pressure 210 to recur and begin the cycle again. This cycling over the course of a sleep session causes disruption in the patient's sleep.
Therefore, it would be desirable to provide an improved auto-adjust algorithm that alleviates these deficiencies in the prior art devices.